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| Joint Astronomy Centre |
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LO Saturation of the 1.3 mm SIS ReceiverDuring the summer and fall 1994 were there several reports of low line intensities using the 1.3 mm SIS receiver RxA2. During normal test observations were we not able to reproduce these low intensities. Nevertheless the tuning was a prime suspect and a test was performed using different LO power levels and magnet currents. The magnet is used to suppress the Josephson super-current. A Josephson super-current can cause severe calibration problems if present. The normal magnet current had been 700 mA to completely suppress the Josephson super-current. However, after cryogenic problems during the Spring the current was decreased to 500 mA. This was to decrease the risk of very explosive boil-off occurring when the magnet coil became non-superconducting. Previous tests had shown that the lower current level did not cause any calibration problems in spite of a weak remaining super-current.
Figure 1. The integrated intensity of the 12CO J = 2-1 in IRC+10216 as function of magnet current for three different mixer currents. Figure 2. The system and receiver noise temperatures as function of magnet current for three different mixer currents. The 12CO J = 2-1 line was observed in IRC+10216. The LO power was adjusted to get 12 mA, 15 mA and 18 mA mixer current while the magnet current was set to 300, 400, 500, 600 and 700 mA. The results are shown in figure 1 and 2. Figure 1 shows the line intensity as a function of the magnet current for the three mixer current settings. A mixer current of 18 mA will cause calibration problems with the magnet set to 500 mA, the curent used during the summer and fall of 1994. Figure 2 shows the system and receiver noise temperatures as functions of the magnet and mixer current. Note that for a mixer current of 18 mA and a magnet current of 300 mA the receiver temperature is negative ! The above results explain the low intensities obtained during the summer and fall of last year. The TO would optimize the tuning and occasionally get a high current level. However, this would not be the normal case. To make sure this not will be repeated the magnet current has now been returned to 700 mA. Further, to simplify the tuning and improve the intensity stability, the mixer current will be set to 15 mA and the mixer bias voltage to 2.05 mV. The reason for calibration differences of 7% between different mixer currents for the same magnet current is not clear. However, setting the mixer current to the given value will increase the stability of the intensity scale with only a small change in performance. Per Friberg / JAC |
